A washer is an annular structure with an opening along its central axis, which fits over the shaft of a threaded fastener, such as a screw or threaded stud. A fastening element, such as the head of the screw or a nut, compresses the washer as it is fastened. The term “washer,” as used in the present patent application and in the claims, refers to any type of annular structure that meets these limitations. Spring washers are designed to apply a pre-load against the fastening element as it is screwed down and thus to prevent subsequent loosening of the fastener. Common types of spring washers include Belleville washers and wave washers.
Washers may be made from shape memory materials, such as Nitinol. For example, U.S. Patent Application Publication 2002/0187020, whose disclose is incorporated herein by reference, describes lock washers that are cut from a sheet or plate of Nitinol. The lock washer remains in its martensitic state in all normal conditions of use. The martensitic Nitinol initially yields during torquing of the nut to allow the nut to indent itself slightly into the washer. The resulting cold working of the washer material causes a transformation into stress-induced martensite, which is strong and elastic to resist further deformation and also exerts a preload on the bolt shank. The nut, indented into the lock washer, resists turning under vibration.
U.S. Pat. No. 3,801,954, whose disclosure is incorporated herein by reference, describes an electrical connector comprising a plug and receptacle coupled together using a screw or bayonet-type coupling and a heat-activatable, generally ring-shaped locking memory member made of a memory alloy, such as 55-Nitinol. The locking memory member is provided with original and intermediate shapes chosen for coaction with predetermined opposed annular surfaces provided on the plug and receptacle so that the application of moderate amounts of heat causes the locking memory member to rigidly lock the plug and receptacle in coupled engagement.
U.S. Pat. No. 4,450,616, whose disclosure is incorporated herein by reference, describes a method of insuring the tightness of a fastener. A three-dimensional annular member is formed in an austenite phase using a press. It is cooled to a temperature lower than the temperature for starting an austenite transformation, and the fastener is fastened with the annular member to flatten the annular member. The temperature of the annular member is then returned to a higher-than-austenite temperature so that the recovery power of the annular member for returning the annular member to the three-dimensional shape in the austenite phase generates a fastening force for the fastener.